Blow molding is a generally known process for molding a preform part into a desired product. The preform is in the general shape of a tube with an opening at one end for the introduction of pressurized gas, typically air. One specific type of blow molding is stretch blow molding (SBM). In SBM applications, a valve block provides both low and high pressure gas to expand the preform into a mold cavity. The mold cavity comprises the outer shape of the desired product. SBM can be used in a wide variety of applications; however, one of the most widely used applications is in the production of Polyethylene terephthalate (PET) products, such as drinking bottles. Typically, the SBM process uses a low pressure supply along with a stretch rod to stretch the preform in a longitudinal direction and radially outward and then uses a high pressure supply to expand the preform into the mold cavity. The resulting product is generally hollow with an exterior shape conforming to the shape of the mold cavity. The gas in the preform is then exhausted through one or more exhaust valves. This process is repeated during each blow molding cycle.
Because of the high pressures flowing into the preform, the valves that control the air flow typically include annular high-pressure seals. One such blow molding valve design is disclosed in international patent publication WO 2008/0426433. While this valve design can generally provide adequate sealing characteristics, the valves are faced with a number of other problems. One problem encountered is during a cleaning process. Due to the various chemicals and impurities that flow through the valves, the valves need to be cleaned periodically. Not only does the exposed valve face need to be cleaned, but the sealed gap that the valve slides along must be cleaned as well. This is because as the valve moves from a closed position to an open position, contaminates are dragged along with the valve seals thereby contaminating the valve piston's bore. As a result, existing blow valves cannot be adequately cleaned on-line. Therefore, existing blow molding valve blocks are required to be shut down and disassembled for cleaning. As can be appreciated, this process is time consuming and costly.
The present invention overcomes this and other problems and an advance in the art is achieved. The present invention replaces the standard seat valves typically used in blow molding valve blocks with a diaphragm valve that substantially isolates the process side of the valve from the control side of the valve. As a result, the valve block can be cleaned on-line without requiring the system to be disassembled. The valves can be cleaned much faster, in some cases the valves can be cleaned in as little as ten minutes.